CryptoClassLoader

CryptoClassLoader.java

  /**
   * This class loader loads encrypted class files.
   */
  class CryptoClassLoader extends ClassLoader
  {
     /**
      * Constructs a crypto class loader.
      * @param k the decryption key
      */
     public CryptoClassLoader(int k)
     {
        key = k;
     }
 
     protected Class<?> findClass(String name) throws ClassNotFoundException
     {
        byte[] classBytes = null;
        try
        {
          classBytes = loadClassBytes(name);
       }
       catch (IOException e)
       {
          throw new ClassNotFoundException(name);
       }

       Class<?> cl = defineClass(name, classBytes, 0, classBytes.length);
       if (cl == null) throw new ClassNotFoundException(name);
       return cl;
    }

    /**
     * Loads and decrypt the class file bytes.
     * @param name the class name
     * @return an array with the class file bytes
     */
    private byte[] loadClassBytes(String name) throws IOException
    {
       String cname = name.replace('.', '/') + ".caesar";
       FileInputStream in = null;
       in = new FileInputStream(cname);
       try
       {
          ByteArrayOutputStream buffer = new ByteArrayOutputStream();
          int ch;
          while ((ch = in.read()) != -1)
          {
             byte b = (byte) (ch - key);
             buffer.write(b);
          }
          in.close();
          return buffer.toByteArray();
       }
       finally
       {
          in.close();
       }
    }

    private int key;
 }
Listing 9-2. Caesar.java
 
  import java.io.*;
 
  /**
   * Encrypts a file using the Caesar cipher.
   * @version 1.00 1997-09-10
   * @author Cay Horstmann
   */
  public class Caesar
  {
    public static void main(String[] args)
    {
       if (args.length != 3)
       {
          System.out.println("USAGE: java Caesar in out key");
          return;
       }

       try
       {
          FileInputStream in = new FileInputStream(args[0]);
          FileOutputStream out = new FileOutputStream(args[1]);
          int key = Integer.parseInt(args[2]);
          int ch;
          while ((ch = in.read()) != -1)
          {
             byte c = (byte) (ch + key);
             out.write(c);
          }
          in.close();
          out.close();
       }
       catch (IOException exception)
       {
          exception.printStackTrace();
       }
    }
 }